Why our Shoelaces don’t stay tied for longer times is a question that puzzles everyone. Thankfully, researchers from the University of California (UC) Berkeley have solved this mystery and found the reason behind shoelaces coming untied mysteriously. According to these researchers, when you walk or run, several forces start working together like an invisible hand to loosen the knot and finally untie your shoelaces.

The primary aim of this study was to gain insight into the mechanics behind loosening of a shoelace bow tie knot. Although earlier studies have tried to explain how sustained loads result in failing of knotted structures, there is little research so far to explain how knotted structures fail under the dynamic forces.

Different ways to tie a shoelace knot

Researchers say a common shoelace bow tie knot can be tied in two ways: the square knot and the false knot. Among them, the square knot version is usually stronger than the false knot version. In a square knot, two lace crossings of opposite handedness remain on the top of one another. On the other hand, in a weaker false knot, the two crossings of the lace have the same handedness, which means the knot is twisted rather than lying flat when tightened. According to researchers, both versions eventually fail in the same way, although their timings are usually different.

Why shoelace knot fails eventually?

In this study, researcher carried out a series of experiments using slow-motion cameras to understand the mechanics behind shoelace knot failure.

In one experiment, the study co-author Christine Gregg laced up running shoes and started jogging on a treadmill, while the experiment was captured on a video camera by another team member. When researchers analyzed the video footage and carried out calculations, they found that the foot, when running, hits the surface at seven times the force of gravity.

The shoelace knot responds to that force by stretching and relaxing, which ultimately loosens the knot. At this time, the free ends of the laces experience an inertial force due to swinging leg, resulting in quick undoing of laces, sometimes in just two strides. The researchers also found that while some shoelaces might work better than others for tying knots, all experienced the same forces responsible for knot failure.

Christopher Daily-Diamond, the co-author of this study, suggests that the understanding of shoelaces and other knotted objects can also be applied to things like microstructures or DNA that are known to have a tendency to fail under dynamic forces.

The detailed findings of the study have been published in the journal Proceedings of the Royal Society A.
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